Many devices transmit electromagnetic radiation when in operation. For example, wireless communication devices intentionally emanate electromagnetic radiation when transmitting. Other devices transmit inadvertently, for example when a microwave oven is cooking, microwaves may inadvertently escape the oven. The widespread acceptance and use of hand-held, portable cellular telephones has been accompanied by increasing concern regarding possible harmful effects of such radiation. New hand-held cellular telephone typically have an elongated housing with an internal antenna, and older hand-held cellular telephones typically have an elongated housing with an antenna extending upward vertically from the housing. When using either type of telephone, the user's head comes into close proximity to the antenna when his head is placed adjacent to the cellular telephone. The antenna emanates radiation when the cellular telephone is transmitting, and such an antenna is referred to herein as a transmitting antenna. Thus, when the user is talking, the device is emanating radiation from the transmitting antenna, and a substantial amount of electromagnetic energy is projected directly onto the user's head at close range.
Each cellular telephone has to meet certain government guidelines as to the amount of radiation the user is exposed to. The amount of RF radiation absorbed by the body is measured in units known as SARs, or specific absorption rates. It would be desirable to reduce the SARs without significantly adversely affecting the operation of the telephone.
There have been attempts to shield the body from the electromagnetic energy emanating from the transmitting antenna. For example, U.S. Pat. No. 5,613,221 issued to Hunt discloses a conductive strip placed between the transmitting antenna and the user's head, to conduct radiation away from the user's head. There have also been some attempts to move the source of electromagnetic energy away from the body by changing the transmitting antenna location or radiation pattern. For example, U.S. Pat. No. 6,356,773 issued to Rinot removes the transmitting antenna from the phone and places it atop the user's head. An insulating shield is disposed between the transmitting antenna and the user's head, like a cap, for blocking emissions so that they do not penetrate through to the user. U.S. Pat. No. 6,031,495 issued to Simmons et alia uses a conducting strip between two poles of a transmitting antenna to create an end fire bi-directional pattern away from the user's head. Others have tried to reduce exposure to harmful emission by canceling the radiation. For example, U.S. Pat. No. 6,314,277 issued to Hsu et alia, is a cellular telephone antenna that cancels transmitted radiation of the cellular telephone with an absorbent directional shield by feeding the signal back into the cellular telephone.
One method of reducing electromagnetic radiation is to capture the radiation with an antenna, convert it to an electric current, and then dissipate the current, as described in U.S. Published Patent Application 2008/0014872. Antennas, however, are designed to receive RF signals in particular frequency bands, and cellular telephones operate generally in one or more of four different bands. For example, in Europe, GSM cellular telephones operate in the 900 MHz and 1800 MHz bands. In the United States, GSM and CDMA cellular telephones operate in the 850 MHz or 1900 MHz bands. It would be desirable to design an antenna for electromagnetic dissipation devices that is capable of capturing radiation across most or all of the cellular telephone frequency bands.
Meander antennas have become popular for receiving cellular telephone signals due to their small size, lightweight, ease of fabrication, and omni-directional radiation patterns. Meander antennas generally comprise a folded wire printed on a dielectric substrate such as a printed circuit board (PCB). Meander antennas have resonance in a particular frequency band in a much smaller space than many other antenna designs. The resonant frequency of a meander antenna decreases as the total wire length of the meander antenna element increases. In addition, if the turns in the meander antenna are very close so as to have strong coupling, there can also be capacitive loading of the antenna, which will increase bandwidth. Total antenna geometry, wire length, and layout must be optimized for each given antenna's purpose. It would be desirable to design a meander antenna for use with an electromagnetic radiation dissipation device that is effective across the cellular telephone frequency bands.
Therefore, it is an object of this invention to provide an antenna design to be used with a device that decreases the SARs to the user of an active emission source without significantly adversely affecting the desired performance of the emission source. It is a particular object to provide an antenna design specifically tuned for reducing the undesirable radiation a user is exposed to from a cellular telephone. It is a further object to provide an antenna design that can capture electromagnetic radiation from a cellular telephone operating in any of the four predominant frequency bands allotted for cellular telephone communication.